The present invention relates to a loop data transmission control method, and particularly to a loop data transmission control system employing multiple control stations which control transmission between a plurality of stations.
The loop data highway system is a transmission system which is widely used in steel and chemical industries for performing production control and computational control for their distributed facilities. In this system, a control station (CST) for transmission control and a plurality of stations (ST) for data transmission and reception are connected through a transmission line to form a loop. Each station (ST) is connected with a computer or a terminal device. In such a system, a breakdown of the CST which controls the overall loop will result in a total system breakdown, and therefore, it is necessary to form a duplex system in some way so that uninterrupted transmission is achieved. An improved duplex CST system which is free from the problems caused by duplicating control stations is disclosed in Japanese Patent Application No. 50-14315 (Patent Laid-Open No. 51-90245).
This known system will be described briefly. In this system, CSTs with independent power supplies are installed separately in arbitrary locations on the transmission loop. One CST is used as an active device (master CST) and another CST is used as inactive device (slave CST). The major reasons for the industrial installation are as follows.
(1) The system is easily controlled and managed upon occurrence of CST mulfunctioning and power loss or local power failure during the maintenance activity for the CST or associated devices, whereby a high security system can be provided.
(2) The CSTs can be installed in arbitrary locations on the loop, thereby allowing a flexible system organization.
The active CST issues suitable transmission command signals for allowing data transmission between the stations (STs). This signal consists of a code section representing a synchronization signal and another code section specifying the address of a station which is allowed to proceed with a transmission. The signal format may differ slightly depending on each transmission system. In any case, however, the transmission command signal is a digital code signal including clock information. On receiving the transmission command signal, the stations (STs) and the control stations (CSTs) extract the clock information from the signal so that it can be used as a clock signal for controlling the processing in each station. For this purpose, each of the STs and CSTs is provided with a phase locked loop circuit (PLL) which operates to oscillate in response to the leading edge or trailing edge of the transmission command signal.
In such a duplex CST system, one station needs to be the master and another station the slave. Two stations cannot act as either the slave or the master at the same time. In the case of both stations being the slaves, the transmission command signal is not issued at all, resulting in a system down condition since transmission is not possible. In the other case of both stations being masters, the unique transmission command signal cannot be obtained and normal transmission does not take place, eventually resulting in a system down condition. Such problems in the duplex CST system are liable to occur when the clock signal on the transmission line is suspended. This can occur when two CSTs take action at substantially the same time to control transmission, resulting in a failure of the master/slave determination. In order to solve the problem caused by such conflict between two CSTs, the above referenced publication proposes to control the CSTs in the following manner.